The present invention relates to a speed change control system of a toroidal continuously variable transmission for vehicles, and in particular to a mechanism which feeds back the gyration angle of power rollers.
A toroidal continuously variable transmission (CVT) for vehicles disclosed by JP-A-H10-148244 published by the Japanese Patent Office in 1998, and JP-A-H7-198015 published by the Japanese Patent Office in 1995, comprises a speed change control valve driven by an actuator and a feedback mechanism which feeds back the gyration angle of power rollers to the speed change control valve.
The actuator, speed change control valve and feedback mechanism are interconnected via a speed change link. The feedback mechanism comprises a precess cam connected to a trunnion supporting one of the power rollers, and a feedback link in contact with the precess cam joined to the speed change link.
The precess cam comprises a cam surface (or a cam groove) having a predetermined inclination (cam lead). The feedback link in contact with this cam surface combines a feedback amount according to the gyration angle of the power roller and a feedback amount according to the axial displacement of the trunnion, and transmits it to the speed change link.
In the above-mentioned conventional technique, the combining ratio of the feedback amount according to the gyration angle of the power roller and the feedback amount according to the axial displacement of the trunnion, is determined by the inclination of the cam surface or the cam groove of the precess cam. This inclination also determines the relation between the drive amount of the actuator which supplies a speed change command, and the variation of the gyration angle of the power roller (approximately equal to real speed ratio).
The axial displacement of the trunnion driven by a hydraulic cylinder is several millimeters, and therefore small. The speed change rate is determined by this axial displacement. To keep the speed change rate constant, it is necessary to increase the axial displacement amount as the speed ratio shifts to the Low side. This is because the peripheral speed of the input disk (speed of the power roller at the point of contact of the power roller and input disk) falls as the speed ratio shifts to the Low side, and the axial displacement amount of the trunnion must be increased to obtain a speed change rate equivalent to the High side. Here, the expression xe2x80x9cthe speed ratio is on the Low sidexe2x80x9d means the side on which the ratio of the output rotation speed to the input rotation speed of the transmission is small, and the speed ratio (reduction gear ratio) of the transmission is large. The expression xe2x80x9cthe High sidexe2x80x9d means the side on which the ratio of the output rotation speed to the input rotation speed of the transmission is large, and the reduction gear ratio of the transmission is small (hereafter idem).
The peripheral speed of the input disk is large on the High side and small on the Low side as described above, so to stabilize gyration angle control at all speed ratios, the inclination of the precess cam must be set small on the High side and set large on the Low side. In other words, the inclination of the precess cam must be varied nonlinearly.
However, when changing the inclination of a precess cam nonlinearly, the variation amount of the gyration angle relative to the drive amount of the actuator is small on the Low side and large on the High side. Therefore, the drive amount of the actuator which supplies the speed change command and the variation amount of the gyration angle are not in a directly proportional relation, i.e., the relation is nonlinear. If the relation between the drive amount of the actuator and the gyration angle is nonlinear, it will be necessary to compensate this nonlinear characteristic by a speed change controller using a map etc., the size of the program will increase, and the capacity of the memory required to store it will also increase.
Further, if the precess cam is assembled in the trunnion with a positional error in the rotating direction, the above-mentioned compensation by the controller will not be performed correctly at all speed ratios, and the precision of speed change control will fall.
It is therefore an object of this invention to make the relation between the drive amount of an actuator and the variation of a power roller gyration angle proportional, and make the feedback amount according to the axial displacement of the trunnion an optimal amount according to the gyration angle of the power roller.
In order to achieve above object, this invention provides a speed change control system which controls a speed ratio of a toroidal transmission including a power roller gripped between an input disk and an output disk, a trunnion which supports the power roller rotatably and displaces in the axial direction, and a fluid pressure cylinder which drives the trunnion in the axial direction. The control system comprises a speed change control valve which controls the fluid pressure supplied to the fluid pressure cylinder, a speed change link, an actuator which drives the speed change control valve via the speed change link, a first feedback mechanism which generates a first feedback amount according to the gyration angle of the power roller, a second feedback mechanism which generates a second feedback amount according to the axial displacement of the power roller and varies a feedback gain according to the gyration angle of the power roller, and a feedback link connected to the first and second feedback mechanisms, which combines the first and second feedback amounts and transmits the combined amount to the speed change control valve via the speed change link.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.